To achieve desirable electricity and functionality, it is usually required to incorporate passive components such as capacitor, resistor, or inductor in a semiconductor package.
As electronic products have been developed toward small size and low energy consumption, the passive components for the electronic products should also be reduced in profile. Conventional insert-type passive components are mounted on a front side of a circuit board in a manner that the circuit board is in advance formed with holes, to allow leads of the passive components to be inserted into the holes and bonded to a back side of the circuit board. However, such passive components have a relatively large size, the leads cannot be closely arranged because of the electrical shortage issue, and the penetrating holes limit the routability of the substrate, making the insert-type passive components gradually replaced by SMT (surface mount technology) passive components.
SMT passive components have a modified structure as compared to the insert-type passive component. FIG. 5 shows a bonding status between a SMT passive component and a substrate. As shown, a pair of bond pads 11′ that are properly spaced apart from each other are formed on a predetermined area (usually around a chip) on the substrate and exposed from a solder mask layer 2′ applied over the substrate. After a proper amount of solder paste (not shown) is applied on the bond pads 11′, two ends 31′ of the passive component 3′ are respectively adhered and secured to the solder paste by a reflow soldering process, allowing the passive component 3′ to be electrically connected to the substrate via the solder paste.
However, since the applied amount of solder paste and the height of the solder paste after being reflow-soldered are difficult to be precisely controlled, and the solder mask layer is hardly achieved with perfect planarity, a clearance of 10 to 30 μm in height is usually formed between the passive component 3′ and the solder mask layer 2′. Such a clearance is dimensionally smaller than the particle size of fillers of an encapsulating resin used for forming an encapsulation body to encapsulate the passive component. Therefore, during a molding process for fabricating the encapsulation body, the clearance underneath the passive component cannot be filled completely by the encapsulating resin. As a result, during a subsequent high-temperature processes such as solder-ball implantation or surface mounting, the solder paste on the two bond pads would be melted under the high temperature and flow into the clearance by capillary action, thereby causing bridging between the two bond pads and short circuit of the passive component, and undesirably degrading the quality and yield of the fabricated products.
Accordingly, U.S. Pat. No. 6,521,997 discloses a substrate formed with a groove thereon, as shown in FIG. 6. The groove is formed through a portion I of the solder mask layer between the pair of bond pads, and is sized to allow the encapsulating resin to pass through the groove. However, the groove has a size limit of a minimum width of 150 μm due to low resolution of the photosensitive solder mask, making formation of such a groove become difficult for a substrate or component that is getting smaller in size.
Current passive components used in BGA (ball grid array) semiconductor packages are mostly of 0603-type or 0402-type, wherein the number indicates the dimensions (length and width) of the passive component, for example of 0402, “04” representing the length (inch), and “02” representing the width (inch). In other words, 0402-type passive component has a length of 0.040 inch (about 1000 μm) and a width of 0.020 inch (about 500 μm), and generally has a thickness of about 500 μm.
As shown in FIG. 7, in the use of a 0402-type passive component for a small semiconductor package, the portion I of the solder mask layer between the two bond pads is 400 μm wide (i.e. the spacing between the exposed bond pads), and provided that the groove is 150 μm wide, a distance between the exposed bond pad and the groove is calculated to be (400−150)/2=125 μm. However, since the semiconductor package is still becoming even smaller in size with the encapsulation body of a current TFBGA (thin and fine ball grid array) package being reduced down to 530 μm in thickness, the 0402-type passive component having a thickness of 500 μm is no longer suitable for the TFBGA package and thus replaced by a smaller 0201-type passive component to comply with the reduced size of the TFBGA package.
The length, width and thickness of 0201-type passive component are all half of those of 0402-type passive component; that is, 0201-type passive component is sized 500 μm long×250 μm wide×250 μm thick. The spacing between the pair of exposed bond pads on the substrate is accordingly reduced to 275 μm; provided that the groove is 150 μm wide, a distance between the exposed bond pad and the groove is calculated to be only (275−150)/2=62.5 μm. The photosensitive solder mask is filled between the exposed bond pad and the groove. However, due to the low resolution of the photosensitive solder mask, which limits the manufacturing accuracy, the solder mask layer is very difficult to be filled in such a small space (62.5 μm wide) between the exposed bond pad and the groove. In other words, the above substrate having the groove and suitable for the 0201-type passive component cannot be fabricated by the conventional technology.
Therefore, the problem to be solved here is to provide a substrate for accommodating 0201-type passive or smaller SMT components without leaving a clearance between the passive component and a solder mask layer on the substrate thereby eliminating the occurrence of electrical bridging between two bond pads on the substrate where the passive component is bonded.